Remember we are only discussing using a fuel based hose for the coolant overflow hose and nothing else. 

Gates fuel hose is compatible with coolant for the over flow reservoir line. There are many articles out there that support this. You can not however use coolant hose for fuel. I've seen it used for many decades for the coolant over flow line. I'm not suggesting it be used as an everyday main coolant hose. Just the over flow from the expansion tank to the reservoir tank. Gates makes regular coolant hose which I use too. i.e. the 25mm (1") coolant hose. It's called Gates Green Stripe coolant hose (it's 1" (25mm) hose). The Gates fuel hose can handle far more chemicals than just coolant. Just look at the fuel people use with up to 20%+ ethanol in some countries and all their additives. It seems that a huge majority of owners and aircraft Mfg's use fuel hose for the reservoir return hose. I work on many LSA aircraft and only a few use something like a polyurethane clear hose.

As Sean also says their are other hose Mfg's out there around the world.

27340 5/16" X 25 FT BARRICADE MPI FUEL

Product # 42196039

Barricade® fuel injection hose is a low permeation, multi-fuel compatible hose that virtually eliminates fuel vapor loss thanks to our 5-layer GreenShield® barrier technology. Works with multiple fuels (approved for use with leaded and unleaded petrol, diesel, biodiesel up to B-100, E-10, E-15, E-85, 100% methanol, ethanol and gasohol fuels).

• Fuel savings from Barricade's incredibly low permeation rate - the lowest on the market at 1g/m2/day.

Hi Sean...for the Rotax coolant specs look at this SI.

We can have a discussion on what is OAT, organic acid technology, and the newer HOAT, hybrid acid technology coolant.  The old green inorganic stuff you should always avoid.  That stuff will fail your water pump seal prematurely.  Section 4 in the SI is what you need to look at.  

Cheers

Hopefully will fly tomorrow and find out if my tinkering solved the problem. Meantime my search for overflow tubing meeting Rotax requirements has yielded this as a possible source of supply, anyone ever used them? https://www.fluorostore.com/products/metric-fractional-pfa-fluoropolymer-tubing?variant=456521493&currency=USD&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&pmaxsmarttop&gclid=CjwKCAjwnOipBhBQEiwACyGLujP5N90OFRx3r9JY5UoMEIPB2y0gI09DB2o58PTI22P1VQyUfeTLXxoCobEQAvD_BwE

Rotax Wizard wrote:

Hi Sean...for the Rotax coolant specs look at this SI.

We can have a discussion on what is OAT, organic acid technology, and the newer HOAT, hybrid acid technology coolant.  The old green inorganic stuff you should always avoid.  That stuff will fail your water pump seal prematurely.  Section 4 in the SI is what you need to look at.  

Cheers

Have perused that SI many times and again just now - still cant find a specification for coolant.

The SI suggests the maintainer go with the aircraft manufactures recommendation - not very helpful to me, I fly a homebuilt Sonex Legacy/Rotax 912ULS

Recommendations for several brands for sure, none of which are readily available in my country (Australia).

Actual specifications would be more useful ie long-life coolant based on monoethylene glycol with advanced organic acid inhibitor technology free from amines, nitrites, phosphates, silicates or other inorganic inhibitors. 

Ok an update. Flew this morning, got her up to temps and returned, shut down and immediately removed top cowl. My overflow bottle had a slight increase in coolant level as compared to takeoff. A few hours later, stone cold engine, rechecked, overflow bottle level was unchanged from my after landing inspection. That is, just a tad higher than what I had at takeoff.